1. Field of the Invention
The present invention relates generally to active noise control systems and methods.
2. Description of Related Art
Technological advances in neonatal intensive care have contributed greatly to decreases in infant mortality. The neonatal intensive care unit (NICU) clinical team must provide support of basic functions including temperature and humidity control, nutritional support, fluid and electrolyte maintenance, respiratory support, and skin integrity management. However, the mission of NICU care is also to support the healthy development of the infant. A critical component of healthy development is limiting the noxious noise to which the patient is exposed while providing appropriate aural stimulation to promote brain and language development. Today, there is no effective solution available for these two facets of developmental care. In the same way that technology has been brought to bear on the physiologic needs through incubators for temperature and humidity management or ventilators for respiratory support, it can also be applied to address these developmental concerns.
Noise levels in NICUs have been shown to be consistently louder than guidelines provided by the American Academy of Pediatrics (AAP). These guidelines stipulate that the noise levels that the hospitalized infants are exposed to should not exceed 45 dB, A-weighted (dBA), averaged over one hour and should not exceed a maximal level of 65 dBA averaged over one second. Noise measured both inside and outside an incubator show guidelines are frequently exceeded throughout the day.
Looking specifically at the sources of noise in the NICU, most are life-critical devices or communication between caregivers, which is often essential for proper care of patients. Specifically, the continuous positive airway pressure (CPAP) device and bradycardia alarms have been reported as between 54 and 89 dBA. Other noise sources include incubator alarms, IV pump alarms, general conversation, telephones, intercom bells, high frequency oscillatory ventilators, televisions, and trolleys or cars. Many of these are essential elements of safe NICU care; their use is not optional, yet they provide a noise hazard to the patient population.
Health risks from noise exposure are many and significant. One growing concern is the indication that NICU noise negatively impacts intellectual development. Hearing loss may be another long-term sequela of NICU noise. It is intuitive that increased noise levels will interfere with the sleep of an infant and this correlation is demonstrated in numerous studies. Adequate sleep is essential for normal development and growth of preterm and very low birth weight infants and can enhance long-term developmental outcomes. Similarly, it has been shown that noise increases various measures of stress in hospitalized infants. Stress is quantified through many surrogates including vital signs, skin conductance, and brow furrowing. While excessive noise is shown to be detrimental to the well-being of the hospitalized infant, proper exposure to human voices, especially in directed communication between parents and the infant, is proving to be beneficial. A correlation exists between the amount of adult language the preterm infant is exposed to in the NICU and the quantity of reciprocal vocalizations and meaningful early conversations.
Active noise control (ANC) may comprise sampling an original varying sound pressure waveform in real time, analyzing the characteristics of the sound pressure waveform, generating an anti-noise waveform that is essentially out of phase with the original sound pressure waveform, and projecting the anti-noise waveform such that interferes with the original sound pressure waveform. In this manner, the energy content of the original sound pressure waveform is attenuated.
Early implementations of this technique were realized with analog computers as early as the 1950s. However, these analog implementations were not able to adapt to changing characteristics of the noise as the environmental conditions changed. With digital technology, adaptive ANC became possible. Sound waves are described by variations in acoustic pressure through space and time where acoustic pressure is the local deviation from atmospheric pressure caused by the sound wave. Incident sound waves can superimpose one upon another in which the net response at a given position and time is the algebraic sum of the waveforms at that point and time. This is known as constructive interference if the resulting pressure is greater than the pressure of any of the incident waveforms and destructive interference if the resulting pressure is less than any of the incident waveforms.
Active noise control can be implemented with a feedforward system employing an upstream microphone that characterizes a sound wave propagating towards a zone. The characterized sound wave acts as a reference signal to an electronic control system that generates a sound wave called a control signal that is essentially 180 degrees out of phase with the reference signal. The control signal is propagated towards the zone and in that zone, the control signal and reference signal interfere with each other. An error microphone is oriented in the zone and measures the sound wave resulting from the interference. This error signal is provided to the electronic control system such that the nature of the control signal can be altered to better reflect the exact opposite of the reference signal. This process continues until the electronic control system converges on an optimum solution to minimize the amplitude of the sound wave in the zone. In this manner, the system is said to be adaptive since the error microphone continuously provides a new signal to the electronic control system as environmental conditions change with the resulting change in the sound wave that propagates towards the zone.
Alternately, active noise control systems can employ a feedback technique. In this approach, a control signal is propagated towards a zone and an error microphone oriented in the zone measures the error signal, which is the response of the sound wave resulting from the interference of the control signal and ambient sound waves that are coincidentally in the zone. The error signal is processed to derive a suitable reference signal to generate a control signal that would better reflect the exact opposite of the coincident sound waves in the zone. This is repeated until the control system converges on an optimum solution to minimize the amplitude of the sound wave in the zone. This system is also adaptive in the same manner as the feedforward system. The feedforward and feedback approaches can be combined into a hybrid feedforward/feedback control system.
Active noise control techniques have been described for use in air ducts to attenuate the emitted sound pressure levels. Applications of duct noise control include: reduction of noise in air conditioning ducts; direction of noise in industrial blower systems; and reduction in vehicular exhaust noise. These can comprise a reference microphone placed upstream in the duct with the control signal being injected downstream to cancel the noise with a feedforward approach. These can also comprise an error microphone placed in the duct essentially at the point of a control source that propagates the control signal into the duct in a feedback approach.
Active noise control techniques have been described in other enclosed space applications. Active headsets have been described and constructed using either feedback or feedforward systems to minimize noise within ear cups of the headset. The small volume of the ear cup facilitates the noise reduction task. The error microphone and control signal source can be placed very close to the ear which improves performance by making the modeling more accurate. Infant incubators have also been described with ANC systems to minimize the noise within the enclosed space of the incubator. The reference microphone is place exterior to the incubator and the control source and error microphone is place within the interior the incubator.
In other applications, ANC systems have been described in other enclosed space situations in which the noise sources are known and predictable and the error microphone can be placed proximate an ear of a user. For instance, a system is described for automobile interiors in which tire sounds are sampled and coupled to a control unit that provides a control signal through a headrest speaker of a car seat. An error microphone within the headrest provides the error signal for the control unit to adapt the control signal. This has the advantage of a physical boundary between the noise source (tires on pavement) and the user's ears on the interior of the automobile. It also has the advantage of a fixed location of the noise source since the tires are permanently fixed to the four corners of the frame of the automobile. Finally, this system can provide for a wired connection between the reference microphone and the control unit, minimizing the transit time between the noise source and the control source.
Applications exist that have been said to be inappropriate for the ANC method. These include reduction of noise within an aircraft cabin or building space and reduction of noise in a space that contains many noise sources that may not be located in predictable positions.